1. Field of the Invention
The present invention relates to a ferroelectric storage device utilizing the reversability of polarization of a ferroelectric.
2. Description of the Related Art
As a ferroelectric nonvolatile memory for storing binary data by utilizing the reversability of polarization of a ferroelectric having a hysteresis characteristic as shown in FIG. 1, a variety of memories are proposed at present.
In such a ferroelectric nonvolatile memory for storing binary data, one bit of digital data of "1" or "0" is stored in each memory cell and the memory capacity corresponds to the number of memory cells.
Accordingly, in order to increase the memory capacity in such a ferroelectric nonvolatile memory, the number of memory cells must be increased. In so far as the degree of integration is not raised, the size of the chip was consequently increased along with the memory capacity.
In recent years, however, along with demands for increasing the capacity of ferroelectric nonvolatile memories, there has been the problem that the memory would become large in size if the number of memory cells were increased along with the increase of the memory capacity.
In view of such a problem, Japanese Unexamined Patent Publication (Kokai) No. 6-196647 discloses a ferroelectric nonvolatile memory capable of increasing the memory capacity without an increase of the chip size by storing multi-level data in a memory cell.
In this ferroelectric nonvolatile memory, each memory cell is provided with a write word line, a read use word line, a write bit line, and a read bit line in addition to a metal-ferroelectric-semiconductor (MFS) transistor, a write use metal oxide semiconductor (MOS) transistor, and a read use MOS transistor. Here, an MFS transistor is a transistor obtained by forming a ferroelectric such as PLZT (Pb,La)(Zr,Ti)O.sub.3) on a gate oxide film.
In this ferroelectric nonvolatile memory, at the time of a write operation, residual polarization based on the analog data or multi-level data is generated in a ferroelectric film of the MFS transistor to directly nonvolatilely store the data. Further, at the time of a read operation, the read MOS transistor is turned on and the drain current based on the residual polarization of the ferroelectric film flowing between the drain and source of the MOS transistor is detected to read the stored data.
In this ferroelectric nonvolatile memory, multi-level data at least trinary or analog data is directly written into or read from each memory cell without the use of an analog-to-digital (A/D) converter, a digital-to-analog (D/A) converter. or other data converter.